1. Field of the Invention
The invention relates to sonar transducers particularly with respect to the velocity measuring type.
2. Description of the Prior Art
Velocity measuring sonar transducers are ubiquitously utilized in correlation velocity log (CVL) sonar systems, doppler ground velocity sonar systems and dead reckoning marine navigation systems. The more precise systems utilize a ground speed measuring transducer which is not subject to ocean currents. Such transducers are typically of the "Janus" type transmitting four narrow beams toward the ocean bottom, each beam being displaced approximately 30.degree. from the vertical. The beams are disposed to radiate from the transducer with each beam separated by 90.degree. from adjacent beams. The transducer is mounted either with the beams directed fore/aft and athwartship or directed 45.degree. with respect thereto.
Such transducers are subject to three principal errors namely scale factor, azimuth misalignment (boresight) and bias. The scale factor error is of a multiplicative type resulting in the measured velocity differing from the actual velocity by a multiplicative factor. The velocity output of the system is corrected by multiplying by an appropriate scale factor. Azimuth misalignment is a function of how accurately the sonar transducer is aligned with the longitudinal axis of the navigation system in which the transducer is utilized. Azimuth misalignment results in cross-track error measured by the cross-axis beams. The bias error is additive and manifests itself in fore/aft and athwartship velocity bias errors. The measured velocity is offset from the actual velocity by an additive factor. The scale factor and alignment errors increase as a function of distance traveled while the bias errors increase as a function of time.
Dead reckoning marine navigations systems utilize such transducers as a means of measuring velocity which is usually resolved into north and east components about ships' heading utilizing data from a compass. The components are then integrated to generate vehicle position.
The types of sonar systems described may be utilized in combination with an inertial navigator which provides continuous velocity and position but must be frequently updated with ground speed from the sonar system in order to maintain accuracy. These precise inertial navigators require that the azimuth alignment of the sonar transducer with respect to the inertial navigator be precisely known in order to avoid errors in the resolution of velocity from body to geodetic coordinates. For example, a 1 milliradian error in azimuth alignment causes a cross-track error equal to 0.1% of distance traveled. Similarly, a scale factor error of 0.1% causes a fore/aft position error equal to 0.1% of distance traveled. In addition to azimuth misalignment and scale factor errors, the sonar fore/aft and athwartship velocity bias errors also contribute to the position error. Thus, in such inertial navigation systems and such sonar navigation systems these parameters must be very precisely determined in order for the systems to provide accurate navigation information.
Manufacturers of such sonar transducers utilize manufacturing and assembly techniques that minimize these errors. The manufacturers, however, only guarantee a scale factor error to .+-.0.2% with no guarantee of azimuth alignment. Typically, azimuth misalignment errors of approximately 5 milliradians occur in production units. These tolerances are an order of magnitude too large to provide the accuracy required in present day navigation systems.
In order to obtain the required navigational accuracy, the scale factor, azimuth misalignment and bias parameters are determined in the prior art as follows. The transducer is calibrated by the manufacturer as part of a calibration procedure involving traversing a known distance, such as a measured mile or longer. This process must be performed at two speeds in order to separate bias from scale factor. Alternatively, the user calibrates each system by traversing an extended distance, such as 30-50 miles, after installation in the vehicle. This process requires the use of a precise position reference and also must be performed by operating at two different speeds. The nominal scale factors are obtained by moving the sonar through the known distance. The scale factor error is obtained by taking the difference between the indicated and actual distances divided by the actual distance traveled. Long distances are required in order to obtain an accurate scale factor. Similarly, misalignment is measured by moving the sonar through a known distance and measuring the cross-track component. Bias errors are typically obtained by operating the sonar at zero vessel speed for a sufficient period of time.
These prior art techniques suffer from the disadvantage of requiring the transducer to traverse large precisely known distances in a vessel utilizing either a precisely measured waterway or a precise position reference. The prior processes are impractical for factory calibration, are slow and tedious, requiring significant supervision and human intervention during calibration. The prior art techniques are also subject to at-sea disturbances, salinity variation, reference position errors, heading errors and human error. The prior art techniques only permit calibration of forward scale factor, it being assumed that the athwartship scale factor is equal to the fore/aft scale factor. When operating in regions of large cross-currents, it is especially important to have an accurate athwartship scale factor which the prior techniques do not provide.